yet another jetting question

This '81 CB650 has something to teach me. (the positive way of looking
at it)

I got it at the beginning of the summer with 13,000 mi. I just checked
the odometer, I put a little more miles on it over the summer than I
thought. It's at 18,000 now.

At first it would cut out if I gave it more than 1/4 throttle, under
load. Going up a hill on the freeway required that I let up on the
throttle so it would have enough power. Downshifting just made it
worse. It seemed like it didn't want to rev, but as long as I built
speed while the throttle was barely open, it would eventually get past
5,000 rpm.

I took the air filter out and it would rev to redline, going uphill
with the throttle open as far as I wanted. The filter was new. I put
it back in and it sucked again.

I cleaned and balanced the carbs a few times. Yesterday I put on some
UNI filter pods. Now it behaves like it should and sounds like a fast
bike.

I'm not sure if I need to rejet. How does a guy determine if it is
necessary.

Also, if anybody cares to speculate, why would a bike with stock
jetting run so much better with UNI filters? I tried setting the idle
mixture to the factory spec, but that's as far as I got, with my
pea-brained tuning.

BTW: The head gasket leaks oil, should I fix that before trying to
solve any other problems?

Thanks for your feedback.

 

Kaybearjr,

Excellent write-up. It's gratifying to hear someone say that a carb mix can
be too rich. Too rich a mix can cause as bad a burn and as poor
performance, as can a too lean mix.

 

This '81 CB650 has something to teach me. (the positive way of looking at

That sounds like something to look into. (in addition to all the other
tips from the group) I don't know what the style of carb is called,
but I know there are no diaphragms, except the accelerator pump
diaphragm. There is a piston with a needle attached that is vacuum
actuated. Now that I have the bike working reasonably well, I notice
that acceleration is jerky, unless I go balls out. It seems like the
pistons might be sticking. Can I polish them, and polish the bores? It
seems like that might help. It seems like trying to lube them would be
a bad idea.

Thanks to all the people that responded to my questions. It's a big
help.

 

Bret:
re: David Kelly's comments:
Your CB650 does have CV carbs, and they do have diaphragms. You can access
them by removing the tops of the carbs (without removing them from the bike,
I imagine). Once the tops are off, you'll see the diaphragms to which Mr.
Kelly is referring. They are large, black, and attached to a metal part
that drops into the barrell of the carb. One easy way to check for holes (if
they are not really obvious) is to shine a flashlight through them in a dark
room.
Check out this diagram: its part 7 on the diagram.

http://houseofmotorcycles.bikebandi...da&model_dept_id=206277&model_dept_name=CB650

Brent.

 

No, those are not diaphragms. The '81 CB650 has the same style of carb that
my '81 CB900 had, they are Keihin CV carbs that use a metal piston to lift
the slide instead of a rubber diaphragm. The diagram isn't especially clear
about that, unless you are familiar with that particular type of carb model.
The distinctive shape of the cylinder on top of the carb body that the
piston slides in, differentiates them from the rubber diaphragm types.

 

That sounds like something to look into. (in addition to all the other

Boy do I feel stupid. I've had these carbs apart several times, and
I've had the nearly identicle ones on my GL500 apart even more. I
never noticed the diaphragms before. Since every diaphragm I've ever
seen has had a hole in it (fuel shutoff, petcock, accelerator pump,
air-cutoff), odds are that this is the case here. If this turns out to
be the case, I'll buy you each a pumpkin pie.

Thanks for the tip on the deal & Happy Thanksgiving.

-Bret

 

That's because your carbs do not have diaphragms...

Hell, if your carbs have diaphragms (except for the air cutoff and accelerator
pump diaphragms) you can buy me a new SV1000S, OK?

 

I would check the fuel level in the carbs
If the level gets to high it will pull more fuel through the main jets
causing the motor to run rich
There are differant ways of checking the level so its best to consult a
manual for this bike
As for the oil leak, if it drips fix it

 

The absolute definitive answer is find someone with a Dyno that knows
how to properly use it. It will measure the gas mixture coming out of
your exhaust whether you are rich or lean and let you know where you are
over the entire power band. If needed it, Air/Fuel mix, can then be
adjusted. It will also give you a true reading for horsepower and
torque. There should be at least 2 runs on the Dyno. First to
establish a baseline and the second to compare. If the two are close to
matching consider that your numbers until you rejet, change the air flow
with a filter, or use a different muffler that is more restrictive or
free flowing with less back pressure. Rejetting will not necessarily
correct an improper air/fuel mix setting. That will depend on your carb
and what adjustments it has and how your manual says to set it up.

Just remember that the bike has to be fully warmed up. Shop temperature
and local altitude will also effect your mix.